DESCRIPTION(provided by applicant): SERPIN biochemistry and catabolism has recently converged with other areas of basic biomedical research, including lipid metabolism and the mechanism of Alzheimer's Disease pathology, as a result of their common interaction with a single biological entity; the low density lipoprotein receptor-related protein (LRP). The low density lipoprotein receptor-related protein is a ubiquitous, 600 kDa cell surface receptor that acts an endocytosis vehicle for a diverse number of ligands. The LRP and its family members have been implicated as playing key roles in the distribution of cell surface proteins, serine protease inhibitor (SERPIN) catabolism, the pathology of Alzheimer's disease, mammalian development, and neuronal cell signaling. In the present studies we will utilize SERPIN:Enzyme complex catabolism as a model system to probe LRP structure/function. We propose to define the quantitative role of the LRP and two of its co-receptors, heparin sulfate proteoglycans (HSPG's) and the urinary plasminogen activator receptor (uPAR) in the differential catabolism of the SERPIN, protease nexin I (PN 1) in complex with different regulatory proteases including thrombin, plasminogen activator and factor XIa. We will also investigate the role of HSPG's in the post-endocytic retention/trafficking of PN1 :Protease complexes, a phenomenon that was recently described in our laboratory. The structural basis for the interaction of the LRP with PN1:Protease complexes will also be investigated to develop strategies for the identification for ligand binding sites in the LRP. This information will be used to construct loss of function genetic variants of the LRP that will be expressed in LRP deficient cells and assayed for biological function. Finally, we will extend our recent observation that PN1 is a potent inhibitor of the blood coagulation protease, FXIa. FXIa has been implicated to play a role in the metabolism of the amyloid precursor protein, and PN1 :FXIa complexes utilize the LRP as clearance receptor. This places PN1, FXIa, APP and the LRP in a common biochemical pathway that may be directly involved in Alzheimer's disease pathology.